The present invention relates to a centrifugal separator for separating particles from gases. The centrifugal separator comprises a vortex chamber, which is provided with at least one inlet for the gases to be purified, disposed in the upper section thereof, at least one outlet for the purified gases, disposed in the upper or lower section thereof, and at least one outlet for the separated particles, disposed in the lower section thereof. At least one vertical vortex is formed in the centrifugal separator.
Various cyclone separators are previously known, which comprise a cylindrical, vertical vortex chamber serving as a separating chamber and having the lower section thereof shaped as a downwardly tapering funnel. The upper section of the vortex chamber is provided with a tangential inlet duct for the gas flow to be treated. The purified gas is generally discharged through an opening disposed centrally at the upper end of the vortex chamber. In flow-through cyclones, the gas is discharged from the vortex chamber through a center pipe disposed in the bottom of the vortex chamber.
In the cyclone, solids are separated from the gases by centrifugal force and they flow along the wall of the separating chamber down to the tapered part of the separator, wherefrom they are discharged. In a conventional cyclone separator, separation is based on the mutual effect of centrifugal force and changes in the flow velocity. The gas flow entering a conventional cyclone starts to whirl spirally, mainly downwardly along the external wall of the vortex chamber, accelerating as the taper diameter becomes smaller. In the lower section of the cyclone, the gases change their direction of movement and start to flow upwardly in the center of the vortex chamber towards the upper section of the separator, which is provided with a gas outlet duct. The solid material concentrated on the walls of the lower section of the vortex chamber by the effect of centrifugal force is not capable of following the gases, but it continues to flow downwardly into an outlet duct.
The cyclone walls are heavily worn especially by abrasive solids. The abrasive effect can be seen particularly in that part of the wall after the inlet which is first hit by the flow of solids. Attempts have been made to decrease the abrasion by protecting the inner surfaces of the vortex chamber by abrasion resistant refractories or by manufacturing the vortex chambers from abrasion resistant materials. High temperature adds to the abrasive effect of solid material.
A problem encountered with circulating fluidized bed reactors, which have become common in combustion and gasification processes, is separating solid particles entrained with hot gas and returning them to the reactor. Special demands placed on the centrifugal separator installed in such a situation are the capabilities to continuously separate great amounts of solids from gases and to endure exposure to erosion when large volumes of hot gases and solid particles flow through the separator.
The main disadvantage with the conventional cyclones in big reactors is that the cyclones have to be heat-insulated, e.g., with ceramic heat insulators for maintaining the outer surface of the separator relatively cold. For providing adequate heat insulation, a thick layer of insulation material is needed, which adds to the price, weight and space requirement of the separator. Furthermore, in order to endure hot conditions, the cyclones have to be internally protected with abrasion resistant layers of refractory. The cyclone walls are thereby covered by two layers of different materials. It is difficult and time consuming to apply these two layers on the walls especially as one of the layers is very thick and has to dry slowly. The two layers are also very susceptible to damage due to temperature differences, e.g. during start-up, and mechanical stress during operation of the system.
On the whole, the cyclone has become an apparatus with thick insulation layers susceptible to damage, which needs a very large space. Because it is a heavy structure, it also requires a strong support structure. This heavy structure means that start-up takes a long time in order to avoid cracks of ceramic parts or refractories. Temperature differences in refractory linings during start-up may cause cracks and, therefore, must be avoided.
The bed material circulating in circulating fluidized bed reactors may be extremely fine, for example, if fine lime is used for absorbing sulfur dioxide in the bed or if the fuel ash is fine. This sets high standards for the cyclone. Attempts have been made to improve the separation efficiency of the cyclone by connecting two or more cyclones in series. Drawbacks of such connections are great pressure losses, expensive structure, and connections requiring much space.
Cyclone batteries comprised of cyclones connected in parallel have also been suggested to achieve a better separating efficiency. The aim has been to achieve higher separation efficiencies by using smaller units. These cyclone batteries are, however, expensive and complicated to manufacture. The cyclone batteries require a certain minimum pressure difference for the gas to be always evenly distributed through the various cyclones.
The walls of the combustion reactors are usually made of water tube panels for partial recovery of the heat generated in the reactor. The cyclone separators and return ducts for the solid material are usually uncooled, heat-insulated structures. Joining such cooled and uncooled parts together is difficult due to unequal heat expansion and thick insulation layers. Therefore, the connections between the reactor and the separator require expensive, ceramic or equivalent heat resistant ductworks and expansion joints. The cyclone separator and the convection section disposed thereafter also require special expansion joints.
When changing the diameter of the cross-section of a cyclone, the distance between adjacent water tubes on the cyclone wall is changed, unless some tubes are taken away or added to some parts of the cyclone wall. This is a complicated process.
For avoiding the above-mentioned drawbacks caused by heat expansion, for example, U.S. Pat. No. 4,746,337 suggests a cyclone of water tube structure. However, manufacturing a cylindrical cyclone of a tubular structure is not simple. Further, the tube panels have to be bent into very awkward shapes in the manufacturing stage, a time consuming and difficult process.
Finnish patent application 861224 discloses a cylindrical cyclone separator of water tube structure, one of the water tube walls being common to both a reaction chamber and a particle separator. As above, this arrangement also involves awkward bends.
U.S. Pat. No. 4,615,715 discloses a separator enclosure manufactured of tube panels and an actual vortex chamber manufactured of a cylindrical, abrasion resistant unit disposed inside the enclosure. The annular space between the separator enclosure and the cylindrical unit is filled with some suitable filler. Due to the cylindrical unit being disposed inside the separator and due to the filler, the separator is, however, large and heavy, although part of the heat insulator has been left out. Furthermore, the cylindrical inner part of the vortex chamber is worn by particles flowing downwards along the walls.
According to the invention, a separator apparatus is provided which is simpler in construction, less susceptible to damages, especially in its insulation layers, does not take up as much space, and is less expensive than conventional high temperature cyclone separators. The centrifugal separator of the invention may be manufactured of simple elements, e.g. mainly planar or plate-formed water tube panels. The separator of the invention is easily made modular. Due to its modular structure, the inventive separator is better applicable than the previously known structures to large circulating fluidized bed reactors, and is highly resistant to abrasion.
It is a characteristic feature of the centrifugal separator according to the invention that the vortex chamber is non-cylindrical, is mainly composed of planar walls, the cross section of the side walls of the vortex chamber preferably being in the shape of a square, rectangle, or other polygon. The cross section of the interior gas space, defined by the vortex chamber, is distinctly non-circular. By "gas space" in a vortex chamber is meant the inner space which can freely be filled up by gas. The gas space is substantially limited by the inner walls of the vortex chamber and by elements fitted on the wall (if there are any). The gas space is a space into which gas can flow freely without being restricted by any elements, refractory layers or the like.
The cross-sectional shape of the gas space of the vortex chamber may be illustrated by a circularity X, which is the circumference of the gas space divided by the circumference of the biggest circle contained in the cross section of the gas space. With a cylindrical separator, X=1, and with a square, X=1.273. In the separator according to the invention, the circularity X of the gas space of the separator is equal to or greater than 1, e.g. X.gtoreq.1.1, and preferably X is equal to or greater than 1.15. While a separator with circularity of X&gt;1 is known per se from German 3435214, such a structure is indicated as being unsuitable for use in separating out particles, and thus teaches away from the invention.
The inside of the vortex chamber of the separator of the invention is at least partially lined with a thin layer of abrasion and heat resistant refractory material. This layer of refractory does not substantially make the cross section of the gas space circular, but it protects areas susceptible to abrasion in the vortex chamber. Nor does the layer of refractory in a preferred embodiment of the invention substantially function as a heat insulator in a vortex chamber. The thickness of the layer of refractory is preferably only about 40 to 150 mm. This thin, abrasion and heat resistant layer of refractory may be attached with studs or other clamping elements to the wall surface of the vortex chamber, said wall surface being preferably a water tube panel. By attaching the layer of refractory directly to a cooled wall, without any insulator or other layers therebetween, cooling of the refractory is also made possible. When cooling, this layer of refractory becomes both chemically and mechanically more durable. Heat-conducting material may be selected as an abrasion resistant material. Such material cools still faster. The studs also enhance cooling. To lessen the abrasive effect of the particles suspended in the inlet gas, the wall opposite to the inlet wall and areas which are especially susceptible to abrasion may be provided with a specific, additional layer of protecting refractory material or with a refractory material which is more abrasion-resistant than the refractory in the rest of the chamber.
In a preferred embodiment of the invention, the walls of the vortex chamber are composed of cooling surfaces, such as water tube panels. Since the vortex chamber is preferably defined by planar walls, the wall elements may be planar or curved ready-made water tube panels. Thus, it is possible to simply assemble a centrifugal separator such as, e.g., a gasification or combustion reactor, by welding it in the intended place of operation. A portion of or preferably all walls of the vortex chamber are of cooled structure. The cooling system of the vortex chamber is preferably connected to the main water/steam system of the fluidized bed reactor with which it is associated.
A cooled particle separator according to the present invention does not have to be lined by thick heat resistant refractory linings or other thick protective layers, which would easily be damaged due to temperature differences during start-up or during operation and, therefore, would easily break or crack [thick linings also consume much space]. According to the present invention, relatively thin abrasion resistant protective layers on the cooling panel are sufficient. According to the present invention, problems with thick linings, as well as other problems due to thermal expansions, can be avoided. Thermal expansions in both reactor chamber and separator can more easily be predicted and compensated when both are formed of water tube panels, where the temperature is more easily controlled. Basically, due to smaller or nonexistent differences in thermal expansion between the reactor chamber and separator according to the invention, problems with expansion joints between the separator and reactor chamber can be minimized.
In accordance with a preferred embodiment of the invention, the centrifugal separator is comprised of an elongated vortex chamber, wherein two or more parallel gas vortices are formed at a spaced relationship. The side walls of the vortex chamber consist of four planar panels, e.g., water tube panels, two opposite walls being the long walls and the remaining two the end walls of the vortex chamber. The long walls may preferably be two or more times longer than the end walls. In that case, the cross section of the inside space of the vortex chamber preferably corresponds to the space of two or more successive squares, the length of a side of the square equalling the length of the end wall. Preferably, the number of gas vortices is equal to the number of squares.
The elongated vortex chamber is provided with a plurality of successive vortices in the longitudinal direction of the chamber by disposing the gas inlet/inlets and the gas outlet/outlets suitably so that the number of vortices produced in the vortex chamber equals the number of the gas outlets therein. The gas outlet/outlets are so disposed in the vortex chamber as to enable the gas from the inlet to be directed tangentially into one or two parallel vortices.
The gas inlets are disposed in the side wall of the vortex chamber so as to guide the gas tangentially from the inlet into the gas vortices in the vortex chamber and maximize the "spin-effect" of the introduced gas jets corresponding to the centers of the gas outlet openings. The "spin-effect" - m * v * r, when m=mass flow, v=gas velocity and r=is the perpendicular distance between gas inlet jet and center of gas outlet opening. The gas vortices formed in the vortex chambers are substantially concentric with the gas outlet openings. It is also possible to guide gas from one inlet to two adjacent gas vortices or guide gas from two or more gas inlets to only one gas vortex.
The elongated vortex chamber is suitable to be disposed next to (in operative association with) a circulating fluidized bed reactor in such a manner that one of the reactor walls or at least part of the upper section of the wall serves as a wall of the vortex chamber. Thus, for example, part of a common long wall of the reactor may serve as a long wall of the vortex chamber, which naturally cuts down the material costs.
Furthermore, two other walls of the reactor may preferably be utilized in connecting the reactor and the separator. The extensions of the walls perpendicular to the common wall may constitute, e.g. the end walls of the vortex chamber. Thus, three cooled panel walls of the reactor may be utilized in the separator construction, which brings remarkable advantages economically and in view of manufacture. This structure enables arranging of, e.g. the combustion furnace of the fluidized bed reactor and the cyclone separator according to the invention so as to compose a single rectangular structure, which is most advantageous with respect to supporting of the structure.
An outlet for the separated solids may be provided corresponding to each gas vortex in the vortex chamber, so that an even distribution of returned solids into the reactor chamber is easy to arrange from several adjacent places, e.g., in a circulating fluidized bed reactor. The solids separated in different vortices may, on the other hand, be collected in one collection chamber or hopper disposed in the lower section of the vortex chamber and may be further conducted to a desired location in one or several particle flows.
In the elongated vortex chamber, the long walls may need support for stiffening the wall panels and for preventing the deflection thereof. In this case, transverse supports or transverse walls may be disposed between the two opposite long walls, for stiffening the chamber structure. The transverse supports/walls are disposed between two gas vortices so that the transverse supports/walls do not have a harmful effect on vortex formation. The transverse supports/walls may be cooled and/or manufactured from abrasion and heat resistant material. The transverse supports may constitute a partition wall in the vortex chamber so as to partly or completely divide the chamber into separate sections. The transverse supports may extend from the ceiling of the vortex chamber down to the bottom thereof, whereby two or, depending on the number of the transverse walls, more completely separate gas spaces are formed in the chamber. On the other hand, the transverse supports may only be short support elements which do not actually divide the chamber into separate gas spaces.
The gas inlets in the vortex chamber are preferably in the shape of vertical, narrow, elongated slots. The slots may be, e.g., as high as the upper section of the vortex chamber. The width of the slot is determined according to the cross-section required for gas flow may be preferably arranged with guide plates for guiding the gas tangentially into the vortex. The guide plates also serve as stiffeners of the long wall.
In centrifugal separators according to the invention only one gas vortex is formed if the separator has a square cross-section. It is easy to arrange a plurality of these parallel and, thereby construct a compact cyclone battery made of simple elements, and taking up little space.
The most significant advantages of the invention are its simple structure and the fact that both a reactor chamber and a small battery of particle separators may be constructed, e.g., of simple, planar parts, such as ready-made water tube panels, which may be manufactured in advance by an inexpensive welding method in a workshop. By arranging a plurality of gas vortices effecting separation of solids in one elongated vortex chamber space, less separator wall area is needed in comparison with a cyclone battery assembled of several independent separators.
Due to cooling, the wall structure of the separator is thinner than that of the conventional hot gas separators and, due to its square/rectangular shape, the separator may be manufactured of plate-formed parts.
A separator according to the invention is constructionally suitable for purifying product or flue gases, for example, in gasifiers and combustion reactors functioning on the fluidized bed principle, where it is desirable to have a cooled structure and where the amount of particles to be separated is great. The invention is especially suitable for separating circulating solids from gases in circulating fluidized bed reactors.